[gdb/tdep] Add vex2_to_vex3

[gdb/tdep] Add vex2_to_vex3

I noticed here [1] that the vex2 prefix is essentially a special case of the
vex3 prefix, meaning it's possible to rewrite any insn with a vex2 prefix into
an equivalent one with a vex3 prefix.

Add function vex2_to_vex3 that does precisely that, in the selftests
namespace.

Add a selftest that exercises this function.

Tested on x86_64-linux.

[1] https://en.wikipedia.org/wiki/VEX_prefix